The present invention relates to an optical writing device serving as an exposer to be incorporated in an image forming apparatus, and a method of manufacturing such an optical writing device.
In a tandem or rotary-type image forming apparatus, it is know that an exposer is embodied by a scanning optical system or a photo emitter array (line head). In the latter case, alignment of photo emitters and lenses is required. For example, Japanese Patent Publication No. 7-186444A discloses that, in order to position an photo emitter array in which a plurality of photo emitters are arrayed and a monocular lens, a mark for indicating a central position of the lens is provided in a lens holder.
In such a line head, it is generally incorporated a one-to-one optical system using a rod lens array unit having two arrays of rod lenses 84 as shown in FIG. 31. In the rod lens array unit, it is necessary to align the photo emitters arrayed in a first direction X corresponding to the primary scanning direction in the scanning optical system with a center line C of the rod lens arrays, relative to a second direction Y corresponding to the secondary scanning direction in the optical scanning system (i.e., the direction that a photosensitive member serving as an image carrier is moved). But misalignment may occur.
In FIG. 31, A represents an example in which the photo emitter array is misaligned from the center line C by 0.1 mm, and B represents an example in which the photo emitter array is misaligned from the center line C by 0.2 mm. As such, when the photo emitter array is misaligned from the center line C of the rod lens arrays, a variation in light quantity occurs. FIG. 32A is a characteristic diagram showing a variation in light quantity in the first direction X, and FIG. 32B is a characteristic diagram showing light quantity profile data of a photo emitter in the second direction Y. As shown in FIG. 32B, when the photo emitter array is misaligned in the second direction Y, the variation in light quantity becomes symmetrical for positive and negative values of a misalignment amount.
In the example of FIG. 31, a diameter of the rod lens 84 is set to 0.56 mm. At this time, if the misalignment between the photo emitter array and the center line C of the rod lens arrays is zero, the variation in light quantity in the first direction X in FIG. 32A is distributed in such a way that a light quantity fluctuation cycle is 0.28 mm which is half of the diameter of the rod lens, like a characteristic Da. When the misalignment amount between them is 0.1 mm, the light quantity fluctuation cycle is the sum of 0.28 mm, which is half of the diameter of the rod lens, and 0.56 mm, which is the diameter of the rod lens. At this time, the light quantity fluctuation cycle is twice as large as that when the misalignment amount is zero. When the misalignment amount of them is 0.2 mm, the light quantity fluctuation cycle is 0.56 mm, which is the diameter of the rod lens.
As such, when the photo emitter array is misaligned from the center line C of the rod lens arrays 65, the following problems occur.
(1) The light quantity fluctuation cycle of light passing through the rod lens becomes large, and fluctuation in light quantity is easily perceived, such that image quality is conspicuously degraded.
(2) The fluctuation in light quantity of light passing through the rod lens is increased.
(3) The light quantity of light passing through the rod lens is reduced.
(4) Imaging performance is degraded, and a spot diameter becomes large or irregular.
In the line head disclosed in the above publication, a line head is fabricated by mounting LEDs serving as the photo emitters on a substrate. In such a case, the photo emitters would not be arranged linearly due to the mounting error, so it is difficult to align the center line of the lens arrays for all photo emitters. In addition, since fluctuation in light quantity of the photo emitter array itself is larger than fluctuation in light quantity of transmitted light of the lens array, in order to correct this problem, a light quantity correction control needs to be performed on each of the photo emitters on the basis of the light quantity of light passing through the lens array, and the fluctuation in light quantity of the photo emitter array itself and the fluctuation in light quantity of transmitted light of the lens array need to be corrected. Further, there is a problem in that the spot diameter cannot be corrected.
In a line head having a plurality of photo emitters, it is important to accurately align the center of the photo emitter with the center of the lens, but various problems may occur, as described above. As described above, in the line head using the LED described in the above publication, a method has been suggested in which marking to be detected is provided so as to indicate a center line for each photo emitter array and a central position for each lens.
In such a method in which marking is provided, the center of the photo emitter array and the center of the substrate are detected, and the position of each lens is adjusted such that the center of the lens is aligned with the centers of the photo emitter array and the substrate. In the method disclosed in the above publication, however, there is a problem in that, when a lens array is used, the adjustment cannot be performed for each lens. Further, in this method, since the central position is detected according to the shape of an electrode, there is a problem in that the shape of the electrode is limited.
Japanese Patent Publication No. 11-138899A discloses a tandem-type image forming apparatus capable of forming a full color image through the use of four colors of toner.
This apparatus incorporates a line head including a photo emitter array 61 in which a plurality of photo emitters 63 are arrayed on a single substrate as shown in FIG. 33. In such a line head, it is generally used a one-to-one optical system including a rod lens array in which a plurality of rod lenses 84 are arrayed.
In this figure, C denotes a center line of the rod lens array, and D denotes a diameter of the rod lens 84. Emergent light of the photo emitter 63 forms a light spot on a surface to be irradiated, such as an image carrier, via the rod lens 84 as shapes of light spots 5 and 6. Here, the light spot 5 has a normal shape having a diameter d, and the light spot 6 has a shape whose diameter is expanded to (d+a) in the first direction X.
The light spot 5 corresponds to a surface to be irradiated by the photo emitters arranged at positions distant from adjacent rod lenses, and the light spot 6 corresponds to a surface to be irradiated by the photo emitters arranged at positions in the vicinity of a boundary between adjacent rod lenses. As such, even when the photo emitters 63 have the same size, the shapes of the light spots in the first direction X are different from each other due to the relative positional relationship of the photo emitter 63 and the rod lens 84 in the first direction X, that is, the position of the rod lens in the first direction X through which emergent light of the photo emitter 63 passes.
The reason will be described with reference to FIG. 34. In FIG. 12A, a horizontal axis represents a distance in the first direction X, and a vertical axis represents a width s of a light spot in the first direction X. D denotes a diameter of the rod lens 84, as described with reference to FIG. 33, and corresponds to a cycle of a fluctuation profile G.
As shown in FIG. 34, in a portion of a bottom of the profile G, the shape of the light spot 5 has a diameter d. Further, in a portion of a top of the profile G, the diameter of the light spot 6 is expanded to (d+a). That is, the shapes of the light spots (spread shapes of light beams) in the first direction X are different from each other by a pitch of the diameter of the rod lens 84. As such, the spread shapes of the light beams in the first direction X are different from each other due to the relative positional relationship of the photo emitter 63 and the rod lens 84. Moreover, there is a problem in that, similarly, the shapes of the light spots (spread shapes of light beams) are different from each other by a pitch of a radius of the rod lens 84.
FIG. 35 is an explanatory view showing an example of the shape of a light spot when a color image is formed. The shapes of the light spots in one line in the first direction X for each color of cyan (C), magenta (M), yellow (Y), and black (K) are shown. As such, the shapes of the light spots of cyan (C) are formed to have different sizes in the first direction X.
For each color of magenta (M), yellow (Y), and black (K), light spots 5 having the normal size and light spots 6 having the large spread shape of the light beam are mixed. In addition, positions where the light spots 6 having the large width of the light beam are formed are different for the individual colors in the first direction X. For this reason, when the colors are superposed by the image forming apparatus described in the above publication so as to form a color image of plural colors, color fluctuation occurs, and image quality is degraded.